Comparative approaches provide unique experimental models and a perspective of biological history of regulation of renal function. Therefore, knowledge obtained from studying lower animal forms should be very applicable towards the understanding of renal function in man in health and disease. The aim of the proposed project is to test the hypothesis that two intrarenal mechanisms, tubulo-glomerular feedback and a simple countercurrent system for concentrating urine, evolved in birds for maintaining osmotic and volume homeostasis. The bird kidneys have features of both primitive animals and mammals in respect to structure and function, and their nephrons, are composed of superficial, short reptilian-type (RT) nephrons and longer mammalian-type (MT) nephrons which possess the loop of Henle. The above hypothesis is based on the following observations: 1) Glomerular filtration rate (GFR) of the birds is more stable than that of primitive vertebrates, and filtration rates of MT and RT nephrons appear to be controlled by different mechanisms. 2) A primitive macula densa evolved in bird kidney. 3) Birds can concentrate urine hyperosmotic to plasma, but the osmotic gradient between the cortex and medulla is small. We intend to pursue for projects in birds: 1) To determine whether changes in NaCl concentration in the peritubular sinusiods alters GFR and renin secretion by a possible tubulo-glomerular feedback evolved specifically in the MT nephrons. Single nephron GFR (SNGFR) of the MT and RT nephrons will be measured. 3) To examine whether vasoactive hormones infulence tubular reabsorption of water and electrolytes, and 4) to determine whether the MT nephrons possess transport and membrane characteristics that provide a basis for a countercurrent multiplier system by using isolated and perfused tubles. Although the ability to concentrate urine of bird kidney is generally poor, some species can live in dry arid or salt-march areas. It is, thus, interesting to investigate renal adaption to environmental changes.